The present invention generally relates to a registration control system for detecting and correcting color misregistration of the images of different colors formed by a plural number of image forming means in a multi-image forming apparatus having a plural number of image forming means, such as a tandem color copying machine or a color printer or an image forming apparatus of the type in which a plural number of images of different colors formed by at least one image forming means are transferred onto a transfer belt, a sheet or an intermediate image transfer medium on a transfer belt, to thereby form a color image. Particularly, the invention relates to an image forming apparatus capable of reducing color misregistration caused by such rotary means as photoreceptor drums of the image forming means and a transfer belt. More particularly, the invention relates to an image forming apparatus capable of reducing an image distortion caused by rotary means, such as photoreceptor drums and a transfer belt, in a black/white image forming apparatus.
Recently, conversion of black/white documents used in offices into color documents rapidly progresses. The image forming apparatus, which handles those color documents, for example, copying machines, printers and facsimile machines, have also rapidly increase their number. In recent office work, there is a tendency of high quality and high speed. With this tendency, the performances of high picture quality and high speed are frequently required for those color document processing apparatuses. To satisfy such a requirement, called tandem color image forming apparatuses have been proposed and some of them have come into practical use and marketed as products. In this type of the apparatus, image forming units are respectively used for the colors, for example, black (K), yellow (Y), magenta (M) and cyan (C). Color images formed by these image forming units are superimposedly transferred onto an image transfer medium or an intermediate image transfer medium being transported, to thereby form a color image.
An example of the tandem image forming apparatus will be described. As shown in FIG. 43, the apparatus includes four image forming units, a black (K) image forming unit 200K for forming a black image, a yellow (Y) image forming unit 200Y for forming a yellow image, a magenta (M) image forming unit 200M for forming a magenta image, and a cyan (C) image forming unit 200C for forming a cyan image. These four image forming units 200K, 200Y, 200M and 200C are horizontally arranged at fixed spatial intervals. An endless transfer belt 202 as a transfer-medium bearing means is disposed under and along the linear array of the black, yellow, magenta and cyan image forming units 200K, 200Y, 200M and 200C. The transfer belt 202 transfers a transfer sheet 201 to the image transferring positions of the image forming units 200K, 200Y, 200M and 200C while electrostatically attracting the transfer sheet 201 thereto.
The constructions of the image forming units 200K, 200Y, 200M and 200C are substantially the same. The units 200K, 200Y, 200M and 200C sequentially form toner images of black, yellow, magenta and cyan, The units 200K, 200Y, 200M and 200C have photoreceptor drums 203, respectively. The surface of the photoreceptor drum 203 is uniformly charged by a primary charging scorotron 204, and exposed to and scanned with an image forming laser beam 205 in accordance with image information. As a result, a latent electrostatic image is formed on the surface of the photoreceptor drum. In the image forming units 200K, 200Y, 200M and 200C, developing units 206 included therein develop the latent images into visual color images with color toner particles of black, yellow, magenta and cyan, respectively. Those toner images are charged by pre-transfer chargers 207, and sequentially and superimposedly transferred onto the transfer sheet 201 that is attracted onto the transfer belt 202 by the charging by transfer chargers 208. The transfer sheet 201 bearing the color toner image consisting of color toner images of black, yellow, magenta and cyan is separated from the transfer belt 202, and subjected to a fixing process by a fixing unit, not shown, to thereby form a color image.
In the figure, reference numeral 209 designates a photoreceptor cleaner; 210, a photoreceptor discharging lamp; 213, a transfer belt cleaner; and 214, a pre-cleaning processing corotron.
The thus constructed tandem image forming apparatus can form a color image at very high speed since a plural number of image forming units are used for forming one image. Where images are formed at high speed, the positioning of the images formed by the image forming units, i.e., registration (abbreviated frequently as regi.) of the color images, frequently loses its exactness. The result is deterioration of the image quality. In this respect, it is difficult to satisfy the requirement of obtaining both high image quality and high image forming speed. The color misregistration is due to the fact that when the temperature within the image forming apparatus varies and an external force is applied to the apparatus, the positions and the sizes of the image forming units per se and the component parts of the image forming units minutely vary. The in-machine temperature variation and the external force application are things inevitable. In ordinary work, for example, removal of jamming, parts exchanging in maintenance, and moving of the machine, an external force is inevitably applied to the apparatus.
As disclosed in the Unexamined Japanese Patent Application Publication No. Hei. 1-281468, for example, there is an proposal of an image forming apparatus which includes a plural number of image forming portions each for forming a visual image defined by image information of an original document (referred simply to as an original frequently hereafter) and a position detecting mark or indicia, and mark detecting means for detecting the position detecting mark formed by each image forming portion and transferred onto a moving member, whereby control of the image forming portions is carried out on the basis of a detecting signal output from the mark detecting means so as to correct an offset of the transferred image misregistered.
A case where the misregistration correcting technique is applied to the called tandem color image forming apparatus shown in FIG. 43 will be described. As shown in FIG. 44, by the four image forming units 200K, 200Y, 200M and 200C of black, yellow, magenta and cyan, color misregistration detecting patterns 220K, 220Y, 220M and 220C, are formed on the transfer belt 202 while being arranged at fixed spatial intervals in the advancing direction of the transfer belt, and color misregistration detecting patterns 221K, 221Y, 221M and 221C are further formed on the transfer belt 202 while being arranged at other fixed spatial intervals in the direction orthogonal to the belt advancing direction. The color misregistration detecting patterns 220K, 220Y, 220M and 220C, and 221K, 221Y, 221M and 221C are sampled in such a way that a line photo sensing device 222, for example, a CCD sensor, including a linear array of a number of photo sensing pixels senses transmitting light emitted from a light emitting device 223 and transmitted through those patterns. The spatial intervals of those color misregistration detecting patterns 220K, 220Y, 220M and 220C, and 221K, 221Y, 221M and 221C are calculated using data gathered by the sampling. The positions and the image forming timings of the image forming units 200K, 200Y, 200M and 200C are corrected by making the calculated intervals equal to the predetermined reference values, to thereby realize a high image quality. The color misregistration detecting patterns 220K, 220Y, 220M and 220C, and 221K, 221Y, 221M and 221C formed on the transfer belt 202 are removed by the transfer belt cleaner 213 after the sampling.
In the color image forming apparatus thus constructed and operated, the following problem arises when the color misregistration detecting patterns are formed on the seam 202a of the transfer belt 202, and some amount of toner of the patterns is left there after insufficient removal by the transfer belt cleaner 213. In the next color image forming process, the residual toner attaches to the reverse side of the transfer sheet 201 held on and carried by the transfer belt 202, to stain the reverse side of the transfer sheet. A minute step is present at the seam 202a of the transfer belt 202. Therefore, it frequently happens that a density of the color misregistration detecting patterns 220 and 221 is varied or the patterns are discontinuous at the seam of the belt. The density variation or the discontinuity of the color misregistration detecting pattern leads to an error in the pattern detection by the line photo sensing device 222.
A sampling correcting method is already proposed by the Applicant of the present patent application (the Unexamined Japanese Patent Application Publication No. Hei. 6-253151). In controlling an image sampling correction process by the registration basis control means of the multi-image forming apparatus, a sampling start point of sampling control means and a sampling width are set, a misregistration measuring pattern is generated repeatedly, and sampling data or arithmetic processing data are added up to obtain a pattern position. An accuracy of detecting the misregistration measuring pattern is improved by, for example, setting the sampling start point of the sampling control means and the sampling width.
The sampling correcting method detects and corrects a color misregistration whose the size and orientation are fixed (referred to as a DC color misregistration), which results from minute variations of the positions and the sizes of the image forming units per se and the component parts in the image forming units, which are caused by an in-machine temperature variation and the application of an external force to the machine.
In addition to the DC color misregistration, there is another type of color misregistration (referred to as an AC color misregistration) whose magnitude and orientation periodically vary. The AC color misregistration is caused by rotation variations of the rotary means, such as the photoreceptor drums and the belt drive roll. The sampling correcting method mentioned above cannot handle the AC color misregistration and further cannot detect the AC color misregistration.
Actually, to cope with this problem, the conventional color image forming apparatus detects a variation of rotation of the rotary means, such as the photoreceptor drum and the belt drive roll, by an encoder attached to the rotary shaft of the photoreceptor drum, for example, and applies a detecting variation of rotation to the related drive motor in a feed forward or a feedback manner, to thereby reduce the variation of rotation.
Even if the control to reduce the rotation variation of the photoreceptor drum in accordance with detecting information derived from the encoder is carried out, other factors cause the AC color misregistration, which deteriorates the image quality. Example of those other factors are an eccentricity of the photoreceptor drum surface caused by the drums per se or its mounting portions, and in some constructions, an eccentricity caused by clearance errors of the rotary shafts of some of the photoreceptor drums, the belt drive roll, and the like. Thus, in the conventional art, the AC vibration component is not contained in an object to be controlled.
The applicant of the present patent application already proposes a unique image forming apparatus as a solution to the problem under discussion (the Japanese Patent Application No. Hei. 7-301381). The proposed apparatus succeeds in considerably reducing the image quality deterioration ensuing from the AC color misregistration caused by the above-mentioned factors by adjusting at least one of the rotation phases of each of the rotary means, such as the photoreceptor drum and the belt drive roll.
The inventor in the above-mentioned patent application explicated the fact that, as shown in FIG. 45A, in the conventional color image forming apparatus, the AC vibration component on one turn of the photoreceptor drum of each image forming unit varies over a fixed region (an image transfer region corresponding to the length of one circumference of each color photoreceptor drum) on the transfer belt, and the waveforms of the profiles representative of the variations of the AC vibration components of the photoreceptor drums are not uniform in phase among the photoreceptor drums. Judging from the fact, it is estimated that in the case of the photoreceptor drums of two colors, K color (black) and Y color (yellow), a phase difference between the waveforms of the profiles of the variations of the AC vibration components of those drums, produces color misregistration as shown in FIG. 45B.
In the invention of that patent application, color misregistration detecting patterns are formed on the transfer belt, periodical rotation phases are picked up from the pattern detecting information, and the rotation phases of the photoreceptor drums, for example, are adjusted by rotation phase adjusting means. In this way, the adverse affects by the rotation variations that will appear on the image are considerably reduced. More specifically, the rotation phase of the K photoreceptor drum is used as a reference phase, and the rotation phases of the photoreceptor drums of the remaining three colors are adjusted on the basis of the reference phase. Actually, the variations of the AC vibration components of the photoreceptor drums, which are not uniform in phase among the drums as shown in FIG. 45A, are actually arranged to be uniform through the rotation phase adjustment, as shown in FIG. 46A. Particularly in this case, the AC vibration components of the respective photoreceptor drums are uniform also in amplitude among the drums. Therefore, the AC color misregistration is completely removed. The AC color misregistration of the K and Y photoreceptor drums, which are caused by the eccentricity of the photoreceptor drums, for example, are reduced to 0 as shown in FIG. 46B.
In the image forming apparatus of the patent application, in case where the waveforms representative of the profiles of the variations of the AC vibration components of the circumferences of the photoreceptor drums of the image forming units are not uniform in phase and amplitude among the drums as shown in FIGS. 47A and 47B, when the rotation phases of the photoreceptor drums are adjusted as in the above-mentioned manner, the vibrations of the AC vibration components of the circumferences of the photoreceptor drums over an image transfer region on the transfer belt are arranged to be uniform in phase among the drums as shown in FIG. 48A. In this case, the amplitudes d of the variations of the AC color misregistration of the K and Y photoreceptor drums are equal to each other (d1=d1). Therefore, the sum of those variations of the AC color misregistration is reduced to zero or therearound. The amplitudes of the variations of the AC color misregistration of the K and M or K and C photoreceptor drums are different from each other. Therefore, the sum of those variations of the AC color misregistration is d1-d2. In other words, the AC color misregistration represented by the difference of (d1-d2) is left after the rotation phase adjustment.
The AC color misregistration arising from the difference between the AC vibration components of those two colors appears on the image as a color misregistration of a perceivable level, to thereby deteriorate the image quality.
If a fine line image, which is formed by superimposing a plural number of colors, suffers from such a color misregistration, the fine line blurs. In a character image formed on a colored white (not white of the sheet) ground, voids appear around the contour of a character. The edge of a colored image area is slightly colored with a color (e.g., magenta or cyan) different from the color of the image area. A seam between the colored image areas looks like a stripe of a different color or a void appears at the seam. In a color ground area, a density periodically varies to form a repeat of bands, viz., a called banding phenomenon occurs.
The inventors of the present patent application found that the following factors causes the AC vibration component, and hence color misregistration and magnification error (image distortion). When the belt drive roll is eccentric, a speed of the moving transfer belt (or the intermediate transfer belt) varies at the transfer points confronted with the photoreceptor drum or a magnification of the image varies in the moving direction of the belt. When the thickness of the transfer belt (or the intermediate transfer belt) per se is not uniform over its length, the belt moves at a varying speed. Specifically, when a thick part of the transfer belt reaches the belt drive roll, the speed of the moving belt is increased. When a thin part of the transfer belt reaches there, the belt speed is decreased. The belt speed periodically varies during a time period of one turn of the belt, resulting in an AC like vibration component. This AC vibration component causes color misregistration or image distortion which appears every one-turn period and every time the thick part of the belt passes the drive roll.